# Buoyancy – floating soap bubbles

A slightly more dramatic demonstration of buoyancy than simply having a series of balls float in water is to float soap bubbles on a layer of carbon dioxide.

Equipment

1. Fish tank
2. Large bag of baking soda
3. Gallon jug of vinegar
4. Soap bubble blower

Demonstration

1. Pour the baking soda into the fish tank and then pour in the vinegar. It will bubble vigorously creating the CO2 layer. You need quite a lot of each.
2. Wait for all the bubbling to stop. You may need to mix it up a bit, but not too much so that you do not mix up the CO2 layer too much.
3. Blow the bubbles into the fish tank. If it all worked out then the soap bubbles should appear to float in mid-air.

Analysis

This is a largely qualitative demonstration, though you can make up some numbers to do a simple calculation. Take a 1 cm radius soap bubble floating in the CO2 layer.

If you assume that the bottom half of the bubble is in the CO2 layer and the top half is in the air then you can calculate the buoyancy force acting on the bubble due to the CO2 and the air

FB=  (4/3)πr3 ((½)ρCO2 g + (½)ρair g)

This is balanced by the weight of the air in the bubble plus the weight of the soap.

W= (4/3)πr3 ρair g + 4πr2soapg

where T is the thickness of the soap film. The balance then becomes

FB= W or (4/3)πr3 (½ρCO2 g + ½ρair g)= (4/3)πr3 ρair g+ 4πr2soapg or (½)r (ρCO2  + ρair )=r ρair + 3Tρsoap